So... one of the disadvantages of being a uniformed officer is that your self defense firearm is very visible - and visible from a distance, as is your uniform. To a lesser extent, this is also a disadvantage of civilian open carry. In an incident being planned by a violent attacker, you would probably be one of the first that they would attempt to disarm. For that reason, a hidden backup handgun makes a great deal of sense.
But carrying two is an additional burden, and you need to be aware of both and they need to stay put.
But one of the advantages of concealed carry is that you won't be automatically identified as the first to neutralize in a crowd. That gives you a considerable advantage. Carrying truly concealed, without the "tells" of self awareness (always favoring or touching the area where your EDC is) keeps you safer.
So, carrying additional magazines fully loaded with ammo makes more sense to me than the weight and bulk of a second backup firearm when I carry concealed. You just must be positive that your EDC is reliable.
My preference is a 9mm Shield or a HK VP9 or P30-SK.